Stressful life events promote anxiety and hypertension and significantly increase the risk for cardiovascular disease which the leading cause of death in the U.S. Developing strategies that limit stress responding to prevent affective and cardiovascular disorders is the long-term goal of this project. Anxiety, hypothalamic pituitary adrenal (HPA) axis dysfunction and enhanced cardiovascular reactivity to stress contribute to the onset of affective and cardiovascular disorders and limiting these indices of stress responding may be therapeutic. Oxytocin (OT) is being used to treat stress-related disorders; however, activation of oxytocin receptors (OTRs) can have diverse effects, and consequently, identifying specific OTRs regulating stress responding may improve treatments. Magnocellular OT neurons are excited by hypernatremia, increases in the plasma sodium concentration (pNa+), and this results in long-lasting increases in central levels of OT. Driving endogenous OT release by elevating the pNa+ in rodents causes activation of OTRs that inhibit corticotropin-releasing-factor (CRF) neurons in the central amygdala (CeA) and paraventricular nucleus of the hypothalamus (PVN). These neurons are implicated in the etiology of anxiety and hypertension, and intriguingly, OTR mediated inhibition of CRF neurons decreased anxiety-like behavior, blunted HPA activation and attenuated cardiovascular responses to stress. Determining the neural mechanism(s) by which this occurs is the main objective of this proposal. In this regard, hypernatremia interacts with stress to increase neuronal activation in the bed nucleus of the stria terminalis (BNST) and the lateral ventral septum (LVS), which express OTRs and connect to brain regions mediating stress responding. Ongoing studies suggest that OTR expressing neurons in the BNST and LVS project to the CeA or function as GABAergic interneurons. These results suggest that hypernatremia activates magnocellular oxytocinergic neurons that release OT in the BNST and LVS, stimulating OTRs expressed on GABAergic neurons that inhibit CRF neurons, thereby attenuating anxiety, HPA activation and cardiovascular responses to stress. Aim 1 will use mice with OTR-specific Cre expression, neuroanatomical tract-tracing and in vitro patch-clamp electrophysiology to test the hypothesis that magnocellular OT neurons send axons that depolarize neurons in the BNST and LVS by activating OTRs. Aim 2 will utilize Cre-inducible adenoassociated virus (AAV) and optogenetics to test the hypothesis that neurons expressing OTRs in the BNST and LVS have GABAergic efferents that inhibit CRF neurons in the CeA and PVN. Aim 3 will delete OTRs in the BNST and LVS of mice to test the hypothesis that these receptors mediate the stress dampening effects of hypernatremia. Completion of these experiments will determine the specific population of OTRs that are activated by endogenously released OT to limit stress responding, thereby producing important information that may guide the development of therapeutics for comorbid affective and cardiovascular disorders.